This invention relates to an improved photovoltaic panel which converts light energy into electrical energy and which exhibits enhanced energy conversion efficiency stability. The device is preferably formed from amorphous-silicon alloys which have improved stability at elevated operating temperatures.
Recently, considerable efforts have been made to develop systems for depositing amorphous semiconductor alloys, each of which can encompass relatively large areas, and which can be doped to form p-type and n-type materials for the production of p-i-n and other type devices which are, in operation in photovoltaic and other applications substantially equivalent to their crystalline counterparts.
It is now possible to prepare amorphous silicon alloys by glow discharge techniques that have (1) acceptable concentrations of localized states in the energy gaps thereof, and (2) provide high quality electronic properties. One such technique is fully described in U.S. Pat. No. 4,226,898, Amorphous Semiconductors Equivalent To Crystalline Semiconductors, issued in the names of Stanford R. Ovshinsky and Arun Madan on Oct. 7, 1980 and by vapor deposition, as fully described in U.S. Pat. No. 4,217,374, issued in the names of Stanford R. Ovshinsky and Masatsugu Izu on Aug. 12, 1980, under the same title. As disclosed in these patents, fluorine introduced into the amorphous silicon semiconductor operates to substantially reduce the density of the localized defect states therein and facilitates the addition of other alloying materials, such as germanium.
The concept of utilizing multiple cells to enhance photovoltaic device efficiency was discussed at least as early as 1955 by E. D. Jackson in U.S. Pat. No. 2,949,498 issued Aug. 16, 1960. The multiple cell structures therein discussed utilized p-n junction crystalline semiconductor devices. Essentially, the concept is directed to utilizing different band gap devices to more efficiently collect various portions of the solar spectrum and to increase open circuit voltage (Voc). The tandem cell device has two or more cells with the light directed serially through each cell, with a large band gap material followed by one or more smaller band gap materials to absorb the light passed through the preceeding cell or layer.
It is of great commercial importance to be able to mass produce photovoltaic devices. Unlike crystalline silicon, which is limited to batch processing for the manufacture of solar cells, amorphous silicon alloys can now be deposited in multiple layers over large area substrates to form solar cells in a high volume, continuous processing system. Continuous processing systems of this kind are disclosed, for the following patents and example, in the following patents and pending patent applications: U.S. Pat. No. 4,400,409 for A Method Of Making P-Doped Silicon Films And Devices Made Therefrom: Ser. No. 244,386, filed Mar. 16, 1981 for Continuous Systems For Depositing Amorphous Semiconductor Material; U.S. Pat. No. 4,410,558 for Continuous Amorphous Solar Cell Production System; U.S. Pat. No. 4,438,723 for Multiple Chamber Deposition And Isolation System And Method; and U.S. Pat. No. 4,492,181 for Method and Apparatus For Continuously Producing Tandem Amorphous Photovoltaic Cells. As disclosed in these applications a substrate formed from stainless steel, for example, may be continuously advanced through a succession of deposition chambers, wherein each chamber is dedicated to the deposition of a specific material.
In making a solar cell of p-i-n type configuration, a single deposition chamber system can be used for batch processing or preferably, a multiple chamber system can be used wherein a first chamber is used for depositing a p-type amorphous silicon alloy, a second chamber is used for depositing an intrinsic amorphous silicon alloy, and a third chamber is used for depositing an n-type amorphous silicon alloy. Since each deposited alloy, and especially the intrinsic alloy must be of high purity, the deposition environment in the intrinsic deposition chamber is preferably isolated from undesirable doping constituents within the other chambers to prevent the diffusion of doping constituents into the intrinsic chamber. In the previously mentioned patent applications, wherein the systems are primarily concerned with the production of photovoltaic cells, isolation between the chambers is accomplished by gas gates through which unidirectional gas flow is established and through which an inert gas may be "swept" about the web of substrate material.
In the previously mentioned patent applications deposition of the amorphous silicon alloy materials onto the large area continuous substrate is accomplished by glow discharge decomposition of the process gases. Among these processes, radio frequency energy glow discharge processes have been found suitable for the continuous production of photovoltaic devices. Also a new and improved process for making amorphous semiconductor alloys and devices has recently been discovered. This process is disclosed in copending application Ser. No. 423,424, filed Sep. 24, 1982 for Method Of Making Amorphous Semiconductor Alloys And Devices Using Microwave Energy. This process utilizes microwave energy to decompose the reaction gases to cause the deposition of improved amorphous semiconductor materials. This process provides substantially increased deposition rates and reaction gas feed stock utilization. Microwave glow discharge processes can also be utilized in high volume mass production of photovoltaic devices as disclosed in copending application Ser. No. 441,280, filed Nov. 12, 1982, for An Improved Apparatus For The Manufacture Of Photovoltaic Devices and to make layered structures as also disclosed in copending application Ser. No. 435,068, filed Oct. 18, 1982 now abandoned, for Method And Apparatus For Making Layered Amorphous Semiconductor Alloys Using Microwave Energy.
It has recently been determined through laboratory experiments that photovoltaic devices having at least one active region formed from an amorphous semiconductor alloy exhibit enhanced energy conversion efficiency stability when operated at elevated temperatures as opposed to the stability of such devices when operated at low ambient temperatures. During the operation of such a photovoltaic device a degradation process can take place wherein over a period of time of operation, the energy conversion efficiency decreases to a point where the efficiency becomes virtually constant. It has been found that the rate of decrease occurs at a slower rate and stabilizes at a significantly higher level when such devices are operated at elevated temperatures above the normal ambient operating temperature.
It is believed that the enhanced stability is obtained because a competing process takes place within such devices when operated at elevated temperatures. This process is an annealing process. Devices which have been operated at a low ambient operating temperature for a sufficient period of time to exhibit a substantially decreased energy conversion efficiency, when annealed at a temperature of, for example, 150.degree. C. for about one hour, exhibit an energy conversion efficiency substantially equal to their initial efficiency. During degradation, a decrease in short circuit current (Jsc) and fill factor (ff) is observed while the open circuit voltage (Voc) remains substantially unchanged. This results in an overall decrease in efficiency. However, after annealing, both Jsc and ff return to their original values.
It is therefore postulated from the foregoing that photovoltaic devices which include amorphous semiconductor alloys when operated at elevated temperatures experience this annealing process in competition with the degradation process. As a result, such devices exhibit enhanced stability when operated at elevated temperatures both in terms of the rate of decrease in efficiency and the stabilized efficiency level.
It is in view of this important discovery that the present invention provides a photovoltaic panel including operating temperature elevating means which maintains the operating temperature of the device at an elevated temperature above the ambient temperature external to the enclosure. As a result, a photovoltaic panel having enhanced energy conversion efficiency stability is obtained.